The present invention relates to deviation and compensation devices for machine tools and, in particular to real time straightness deviation measuring and compensation devices for machine tools.
Machine tools are utilized in a wide variety of manufacturing processes and must be able to move through the three mutually perpendicular axes of space with great precision, accuracy, and reliability. In three-dimensional space, the degree of precision, accuracy, and reliability of the machine tool obtainable in the real world is the extent to which its three axes remain perpendicular and straight relative to each other and to the workpiece.
The prior art obtained and maintained relative axes perpendicularity and straightness by fabricating rigid way supports of cast or weldment construction superimposed on a rigid steel reinforced concrete foundation. This provided a secure mounting structure on which were mounted scrapped V-ways or roller ways providing the moving contact with the machine tool carriage. In order to obtain the best possible perpendicularity and straightness, and allow for the adjustment of the ways, adjustment devices such as leveling wedges, side pushers, and hold-down bolts were incorporated in the machine base. These adjustment devices were usually adjusted at installation using a laser interferometer or a wire micrometer to provide perpendicularity and straightness. However, with the prior art adjustment devices it is initially difficult to adjust the bed section using wedges and jacks to obtain movement in the exact magnitude, location, and direction desired. In addition, over time the accuracy of the initial adjustment degrades due to a variety of influences such as settling, temperature change, cutting forces, accidents, and the like. It is thus necessary because of the variety of influences to periodically readjust the machine tool in order to correct and maintain its perpendicularity and straightness.
With the advent of CNC control systems, bed error compensation became available whereby a one-time map of the straightness error was generated by obtained data and entered into a computer. A servo system would compensate the cutting tool to obtain the desired results according to the generated map, thus taking into account the bed error. This worked fairly well when the machine was new. However, since the effects of time generally degrade the validity of the generated bed error data and associated map, the complex process of obtaining new data and generating a new bed error map would have to be repeated.
While the prior art conventional devices are good for many applications, the machine tool, in order to be reliably accurate, must also maintain perpendicularity and straightness through four-dimensional space; that is, in real time. Prior art devices, such as CNC control systems and rigid structural foundations with manually adjustable wedges and jacks, do not compensate for deviations such as cutter/cutting forces, temperature change, and vibration that occur as the machine tool is operating in real time. Because of this, exact perpendicularity and straightness is not maintainable during the period during which the machine tool operates. As an example, bed way deviations from a truly straight path was not heretofore compensatable in real time.
Additionally, the prior art conventional devices are not useful in applications where straightness in the order of a few microns must be maintained over long distances, e.g. 8 to 12 meters or more, such as in roll grinding.